Mr. J. Alder on the British Tunicata. 159 



fully bearing out the propriety of its separation from the allied 

 genera Asctdia and Cynthia, between which it is somewhat in- 

 termediate, and though more nearly allied to the latter, yet at 

 the same time possessing characters distinct from both. Where 

 external characters can be found to distinguish genera and spe- 

 cies, it is alw ays desirable that they should be adopted in descrip- 

 tion, though, of course, the whole structure of the animal must 

 be studied in order to assign it its proper place in a natural 

 arrangement. To any one conversant with the Tunicata, it must 

 be obvious that external characters are often insufficient to de- 

 termine the species, or sometimes even the genus, of many of 

 the simple Ascidians. Internal characters must therefore be 

 resorted to ; and of these the structure of the branchial sac and 

 the tentacular filaments are not only of most importance, but of 

 most easy access for examination. The branchial sac of Molgula 

 is very peculiar, the meshes being convoluted in an irregular 

 manner, differing from any other with which I am acquainted. I 

 propose, therefore, to re-define Molgula in the following manner : 

 Animal generally free, or only slightly attached by glandular 

 hairs. Test thin and membranous, often covered with sand or 

 fragments of shells, very slightly attached to the mantle except 

 at the two apertures. Branchial aperture 6-lobed, anal 4-lobed ; 

 ocelli inconspicuous or none. Tentacular filaments branched. 

 Branchial sac with longitudinal folds ; the meshes more or less 

 convoluted, without papillae. Ovaries on both sides of the body, 

 that on the right situated outside the flexure of the intestine. 

 Stomach and intestine lateral and dextral, the latter bending 

 upwards as in Ascidia, but with the flexure more compressed. 



Molgula socialis, n. sp. 



Body ovate, covered with fine sand, adhering by a small base. 

 Apertures terminal, approximated, rather small, tubercular. 

 Test greenish, thin, soft, covered with longish, unbranched, 

 rather rugged, glandular hairs. Mantle greenish, soft. Tenta- 

 cular filaments large, much branched, tripinnate. Branchial sac 

 with six folds on each side, the meshes irregular and imperfectly 

 convoluted. Height about half an inch. Densely gregarious. 



For a knowledge of this species I am indebted to Dr. Bower- 

 bank, who obtained it from the fishermen at Hastings. The 

 specimens were attached to Pecten maximus, from the Diamond 

 trawling-ground, about twelve miles from that place. 



Unlike the other species of this genus, which are generally 

 solitary, this Molgula is associated in dense masses, firmly ad- 

 hering to each other, and so closely as often to press the sides 

 into a square or hexagonal form. The tentacular filaments are 

 large in proportion to the animal, and beautifully arborescent. 



